Trenched schottky rectifiers

ABSTRACT

Inner trenches ( 11 ) of a trenched Schottky rectifier ( 1   a;    1   b;    1   c;    1   d ) bound a plurality of rectifier areas ( 43   a ) where the Schottky electrode ( 3 ) forms a Schottky barrier  43  with a drift region ( 4 ). A perimeter trench ( 18 ) extends around the outer perimeter of the plurality of rectifier areas ( 43   a ). These trenches ( 11, 18 ) accommodate respective inner field-electrodes ( 31 ) and a perimeter field-electrode ( 38 ) that are connected to the Schottky electrode ( 3 ). The inner field-electrodes ( 11 ) are capacitively coupled to the drift region ( 4 ) via dielectric material ( 21 ) that lines the inner trenches ( 11 ). The perimeter field-electrode ( 38 ) is capacitively coupled across dielectric material ( 28 ) on the inside wall ( 18   a ) of the perimeter trench  18,  without acting on any outside wall ( 18   b ). Furthermore, the inner and perimeter trenches ( 11, 18 ) are closely spaced and the intermediate areas ( 4   a,    4   b ) of the drift region ( 4 ) are lowly doped. The spacing is so close and the doping is so low that the depletion layer ( 40 ) formed in the drift region ( 4 ), from the Schottky barrier ( 43 ) and from the field-relief regions ( 31,21; 38,28 ) in the blocking state of the rectifier, may deplete the whole of the intermediate areas ( 4   a,    4   b ) between the trenches ( 11, 18 ) at a blocking voltage just below the breakdown voltage. This arrangement reduces the risk of premature breakdown that can occur at high field points in the depletion layer ( 40 ), especially at the perimeter of the array of rectifier areas ( 43   a ).

[0001] This invention relates to Schottky rectifiers, and moreparticularly to measures for increasing the breakdown voltage of suchrectifiers. The invention also relates to methods of manufacturing suchrectifiers.

[0002] Schottky rectifiers are known comprising a semiconductor bodyhaving a body portion of one conductivity type between first and secondmain electrodes, of which the first main electrode forms a Schottkybarrier with the body portion at a plurality of rectifier areas of afirst surface of the body portion. Various embodiments of suchrectifiers are disclosed in United States patent U.S. Pat. No. 4,646,115(our reference PHB33047), the whole contents of which are herebyincorporated herein as reference material. In one type of embodiment, apattern of trenches extends into the body portion from the firstsurface. The pattern comprises inner trenches that bound each rectifierarea and a perimeter trench that has an inside wall extending around theouter perimeter of the plurality of rectifier areas. The trenchesaccommodate a field-electrode that is connected to the first mainelectrode. The field-electrode is capacitively coupled to the bodyportion via dielectric material that lines the trenches so as to providefield-relief regions in the body portion.

[0003] The inner trenches are sufficiently closely spaced and theintermediate areas of the body portion are sufficiently lowly doped thatthe depletion layer formed in the body portion (from the Schottkybarrier and from the field-relief regions in the blocking state of therectifier) depletes the intermediate areas of the body portion betweenthe trenches at a voltage less than the breakdown voltage. In thismanner, the trenched inner field-relief regions significantly improvethe voltage blocking characteristic of the device.

[0004] Premature breakdown of this type of Schottky rectifier can occurat high field points in the depletion layer, especially at the perimeterof the active area. To reduce or avoid such premature breakdown, U.S.Pat. No. 4,646,115 discloses providing this type of rectifier with aperimeter field-relief region comprising a field electrode on dielectricmaterial in a perimeter trench. U.S. Pat. No. 4,646,115 describesforming the perimeter field-relief region simultaneously with the innerfield-relief regions so as to reduce the total number of processingsteps for the manufacture of the device. In the embodiments shown inU.S. Pat. No. 4,646,115, the perimeter trench is of the same depth andwidth as the inner trenches. It is lined with the same thickness of thesame dielectric material. The perimeter field electrode is present onthis dielectric material on inside and outside walls of the perimetertrench (as well the bottom of the trench) and so is capacitively coupledto the body portion across both the inside wall and the outside wall.

[0005] It is an aim of the present invention to improve the trenchedfield-relief regions of Schottky rectifiers, especially at the perimeterof the device, and to facilitate the manufacture of these improvedrectifiers.

[0006] According to the present invention, there is provided a Schottkyrectifier with trenched inner and perimeter field-relief regions. Theperimeter field-electrode in its perimeter trench is present on itsdielectric material on the inside wall of the perimeter trench so as tobe capacitively coupled across said inside wall without acting on anyoutside wall. Furthermore, the inner and perimeter trenches aresufficiently closely spaced and the intermediate areas of the bodyportion are sufficiently lowly doped, that the depletion layer formed inthe body portion in the blocking statue of the rectifier depletes theintermediate areas of the body portion between the trenches at a voltageless than the breakdown voltage. Advantageously the perimeter trenchextends deeper in the body than the inner trenches to improve itsinwardly directed field relief function.

[0007] Thus, in a rectifier in accordance with the invention, theinwardly-acting field electrode of the perimeter trench is soconstructed and arranged with respect to the inner trenches as to reducethe high field points by depleting the body portion between thetrenches, without any significant outward extension. This depletionarrangement uses the perimeter and inner trenched field-electrodes in aparticular form of the so-called “RESURF” technique. Particularadvantageous forms of this construction and arrangement can be achievedwithout requiring extra processing steps in manufacture. In particular,the perimeter trench can be made deeper than the other trenches bymaking it wider. Due to local loading effects during etching of theinner trenches, this increased width can be used to produceautomatically a deeper perimeter trench. A thick dielectric layer isadvantageous in the deep perimeter trench and can be provided in variousways.

[0008] The invention may be advantageously used in conjunction withvarious known Schottky rectifier options. Thus, for example, a gradeddoping can be advantageous in the body portion in some situations, asdescribed in United States patent U.S. Pat. No. 5,612,567 and in pendingU.S. patent application Ser. No. 09/167,298 which is referenced incolumns 11 & 12 of U.S. Pat. No. 5,998,833. The whole contents of U.S.Pat. No. 5,612,567, U.S. Pat. No. 5,998,833 and U.S. application Ser.No. 09/167,298 are hereby incorporated herein as reference material. Asdescribed in U.S. Pat. No. 5,998,833, U.S. application Ser. No.09/167,298 also describes the use of breakdown shielding regions betweenthe perimeter trench and the inner trenches.

[0009] Some of the particularly advantageous technical features and someof the options available with the invention are set out in the appendedclaims. The invention provides several advantageous novel combinationsof features, many of which are illustrated in the embodiments now to bedescribed with reference to the drawings. Specific examples are thedepth and width of the perimeter trench and its relationship to theperimeter of the semiconductor body, and adjustments in the dopantconcentration of the body portion in relation to an increase ofdielectric thickness in a lower part of the trench.

[0010] Particular embodiments of the present invention are nowdescribed, by way of example, with reference to the accompanyingdiagrammatic drawings, in which:

[0011]FIG. 1 is a cross-sectional view of part of a trenched Schottkyrectifier in accordance with the invention;

[0012]FIGS. 2, 3 and 4 are similar cross-sectional views of part ofthree other trenched Schottky rectifiers also in accordance with theinvention;

[0013] and FIG. 5 is a cross-sectional view of part of a semiconductorwafer comprising two body parts of FIG. 2 or 3 or 4, at a stage in themanufacture of the rectifiers of FIG. 2 or FIG. 3 or FIG. 4 by a methodin accordance with the invention.

[0014] It should be noted that all the Figures are diagrammatic.Relative dimensions and proportions of parts of the drawings have beenshown exaggerated or reduced in size, for the sake of clarity andconvenience in the drawings. The same reference signs are generally usedto refer to corresponding or similar features in modified and differentembodiments.

[0015] The Schottky rectifiers 1 a, 1 b, 1 c and 1 d of FIGS. 1 to 4each comprise a semiconductor body 10 having a body portion 4 of oneconductivity type (n-type in this example) between first and second mainelectrodes 3 and 34. The first main electrode 3 forms a Schottky barrier43 with the body portion at a plurality of rectifier areas 43 a of afirst surface 10 a of the body portion 4.

[0016] A pattern of trenches 11, 18 extends into the body portion 4 fromthe surface 10 a. This pattern comprises inner trenches 11 that boundeach rectifier area 43 a and a perimeter trench 18 that has an insidewall 18 a extending around the outer perimeter of the plurality ofrectifier areas 43 a. The trenches 11 and 18 accommodate respectiveinner field-electrodes 31 and a perimeter field-electrode 38 that areconnected to the first main electrode 3 of the rectifier 1 a, 1 b, 1 c,1 d. These field-electrodes 31 and 38 are capacitively coupled to thebody portion 4 via dielectric material 21 and 28 that lines therespective trenches 11 and 18 so as to provide field-relief regions inthe body portion 4.

[0017] In each of the Schottky rectifiers 1 a, 1 b, 1 c and 1 d of FIGS.1 to 4, the field-electrode 38 in the perimeter trench 18 is present onthe dielectric material 28 on the inside wall 18 a of the perimetertrench 18. It is capacitively coupled across this inside wall withoutacting on any outside wall. The perimeter field-electrode 38 is notpresent on any dielectric material lining any outside wall 18 b of theperimeter trench 18 and is not capacitively coupled to the body portion4 across any such outside wall 18 b. Indeed the perimeter trench 18 hasno outside wall in the rectifiers 1 b, 1 c and 1 d of FIGS. 2 to 4.

[0018] Furthermore, in each of the Schottky rectifiers 1 a, 1 b, 1 c and1 d of FIGS. 1 to 4, the intermediate areas 4 a and 4 b of the bodyportion 4 are lowly doped and the inner and perimeter trenches 11 and 18are closely spaced. The doping of the areas 4 a and 4 b is sufficientlylow that the electrode 3 can form the desired Schottky barrier 43 withthe body portion 4. Furthermore, the spacing is so close and the dopingis so low that the depletion layer 40 formed in the body portion 4 (fromthe Schottky barrier 43 and from the field-relief regions 31,21 and38,28) in the blocking state of the rectifier depletes the whole of theintermediate areas 4 a and 4 b of the body portion 4 between thetrenches 11 and 18 at a voltage less than the breakdown voltage. Thedepletion may occur at or near the maximum blocking voltage of thedevice, which is near, i.e. just below, the breakdown voltage.

[0019] Thus, in the rectifiers in accordance with the invention, theinwardly-acting field electrode 38 of the perimeter trench 18 is soconstructed and arranged with respect to the inner trenches 11 as toreduce high field points by depleting the body portion 4 between thetrenches 18 and 11, without any significant outward extension of thedepletion layer 40. Apart from the construction and arrangement of thisinwardly-acting field electrode 38 and the close spacing of the trenches18 and 11, the rectifiers 1 a, 1 b, 1 c and 1 d of FIGS. 1 to 4 can beknown type. Thus, the rectifiers 1 a, 1 b, 1 c and 1 d of FIGS. 1 to 4may be manufactured with similar geometries, materials, processes, anddoping concentrations to those described in U.S. Pat. No. 4,646,115,U.S. Pat. No. 5,612,567, U.S. Pat. No. 5,998,833 and U.S. applicationSer. No. 09/167,298. Advantageous novel differences in accordance withthe invention may also be adopted as disclosed hereinafter.

[0020] Most usually, a Schottky rectifier in accordance with theinvention will be a discrete vertical device structure such as isillustrated in FIGS. 1 to 4, in which the second main electrode 34 is atthe bottom surface 10 b of the body 10, where it forms an ohmic contactwith a highly doped (n+) substrate 60. Typically, the device body 10 isof monocrystalline silicon. The doping concentration (n+) of thesubstrate 60 may be, for example, 10¹⁸ to 10²¹ phosphorus or arsenicatoms cm⁻³. Aluminium or Ti—Ni—Ag are two examples of commonly-usedelectrode materials suitable for the ohmic-substrate electrode 34. Onthis substrate 60, an epitaxial layer of higher resistivity is presentto provide the body portion 4 with which the Schottky barrier 43 isformed. The epitaxial layer and substrate are of the same conductivitytype, usually n-type. The choice of material for the Schottky electrode3 depends on the desired barrier height, and specific examples ofsuitable commonly-used materials are platinum silicide or titanium. Thechoice of doping concentration and thickness for the drift region 4depends on the desired blocking voltage of the rectifier, but is usuallyin the range of, for example, 10¹⁵ to 10¹⁷ phosphorus or arsenic atomscm⁻³ with a thickness of about 2 μm (micrometers) or more. The driftregion 4 may have a uniform doping concentration (n), for example of theorder of 10¹⁵ dopant atoms cm⁻³. However, as described below, the driftregion 4 may have a doping concentration (n) that increases with depthin order to reduce the on-resistance of the device.

[0021] The inner field-electrodes 31 can be formed conveniently ofconductive polycrystalline silicon on an insulating layer 21 of silicondioxide. The perimeter trench dielectric 28 may also be of silicondioxide, and may even have the same composition and thickness(es) as thelayer 21 of the inner trenches 11. The perimeter field electrode 38 canbe formed conveniently of the same material as the Schottky electrode 3or the inner field-electrodes 31. By way of example, FIGS. 1 to 3 showthe perimeter field-electrode 38 formed by a simple extension of theSchottky electrode 3. FIG. 4 shows the perimeter field-electrode 38formed by extending the inner electrode network 31 outward, around theperimeter wall 18 a. A similar extension of the inner electrode network31 may also be adopted to form the perimeter field plate 38 in amodification of the devices of FIGS. 1 to 3.

[0022] Usually the inner trenches 11 are sufficiently deep to extendacross most of the thickness of the drift region 4. The trenches 11 mayeven extend slightly into the substrate 60, a specific example beingshown in FIG. 3. The depth of the perimeter trench 18 may be about thesame as that of the inner trenches 11, or it may be deeper. The closespacing of the inner trenches 11 and perimeter trench 18 may be such asto provide a width of, for example, 0.5 μm to 1 μm for the intermediateparts 4 a and 4 b of the drift region 4. Thus, if the width of the innertrench 11 is 0.5 μm to 1 μm, then the trenched rectifier has a cellpitch of 1 μm to 2 μm, i.e. a spacing of 1 μm to 2 μm between centres ofthe neighbouring trenches 11.

[0023] In a blocking state of the rectifier, a depletion layer 40 isformed in the drift region 4 from the Schottky barrier areas 43 a withthe drift region 4 and from the field-relief regions 31,21 and 38,28.The extent of this depletion layer 40 is indicated in chain dot outline(

) in FIGS. 1 to 4. Thus, the depletion layer 40 of FIGS. 1 to 4 extendsacross the whole of the drift region 4 between the trenches 11,18 andalso slightly into the higher-doped substrate 60. This depletion layer40 depletes the whole of the intermediate areas 4 a of the drain driftregion 4 between neighbouring trenches 11 at the blocking voltage. Thisis caused by a field plate effect of the trenched field-electrode 31 ofthe neighbouring cells in the drift region 4.

[0024] At the edge of the rectifier, a field plate effect is achieved bythe provision of the field electrode 38 around the array perimeter in amanner in accordance with the invention. This electrode 38 iscapacitively coupled across the dielectric material 28 in the perimetertrench 18, but only on its inside wall 18 a without effectivelyextending as a field plate on any outside wall 18 b of the perimetertrench. Thus, the field electrode 38 acts inwardly towards the rectifierarray, without significantly spreading the depletion layer 40 outwardlytowards the perimeter 15 of the semiconductor body 10. The resultingdepletion of the intermediate area (4 b in FIGS. 1 and 2 and 4 a in FIG.3) between the trenches 11,18 reduces the electric field around theperimeter of the outermost active cell 1, while avoiding any breakdowntowards the perimeter 15 of the body 10.

[0025] Many modifications and variations are possible within the scopeof the present invention. Several such modifications are illustrated inthe separate embodiments 1 a, 1 b, 1 c, and 1 d of FIGS. 1 to 4. It willbe evident that alternative features which are shown in one embodimentmay be adopted in another of the embodiments.

[0026] In the rectifiers of FIGS. 1 to 4, the perimeter trench 18extends deeper in the body 10 than the inner trenches 11 and is widerthan the inner trenches 11. Since the electric field at the bottom ofthis deep trench 18 is larger than at the bottom of a shallower trench,the dielectric 28 that lines at least the lower part of this deep trench18 is preferably quite thick. Thus, it can be advantageous for at leastthis area of the dielectric 28 to be thicker than the dielectric 21 thatlines at least an upper part of the inner trenches 11.

[0027] In the rectifier 1 a of FIG. 1, the deeper and wider trench 18 isspaced from the perimeter 15 of the semiconductor body 10 by aperipheral area 4 c of the drift region 4. Thus, this perimeter trench18 has an outside wall 18 b, as illustrated in FIG. 1. Although FIG. 1shows (in broken outline) a possible dielectric layer 28 a on thesurface 10 a of the peripheral area 4 c, this dielectric 28 a can beomitted. Similarly, even the dielectric layer 28 b on the outside wall18 b of the perimeter trench 18 could be omitted. The omission of thesedielectric layers 28 a and 28 b is possible because of the peripheralisolating effect of avoiding any field plate action in an outwarddirection from the trench 18 towards the perimeter 15 of the body 10.This peripheral isolation can still be achieved if the gap shown in theperimeter trench 18 in FIG. 1 were to be filled with an insulatingmaterial of sufficiently low dielectric constant and large thicknessthat there is no significant capacitive coupling between the field plate38 and the peripheral portion 4 c at the outside wall 18 b.

[0028] However, it is not necessary for the perimeter trench 18 (thatextends around the array of rectifier areas) to be spaced from theperimeter 15 of the body 10. FIGS. 2 to 4 illustrate specificembodiments 1 b, 1 c and 1 d having a simpler and more compact layoutgeometry. In the rectifiers 1 b, 1 c and 1 d, the perimeter trench 18 isso wide as to extend to the perimeter 15 of the body 10. Thus, thesedevices of FIGS. 2 to 4 have no outside wall 18 b to their perimetertrench 18.

[0029] The perimeter trench 18 of FIGS. 2 and 3 is so deep as to extendthrough the thickness of the drift region 4 to the higher conductivitysubstrate 60. The perimeter trench 18 in the devices of FIGS. 1 and 4may likewise extend into the substrate 60, or it may be shallower, forexample even of the same depth as the inner trenches 11. FIGS. 1 and 4illustrate an intermediate situation where the trench 18 is deeper thanthe inner trenches 11 but shallower than the interface of the driftregion 4 with the substrate 60. This intermediate situation may also beadopted in a modification of the devices of FIGS. 2 and 3.

[0030]FIG. 1 illustrates a rectifier in which the dielectric 28 in theperimeter trench 28 is of the same composition and thickness as thedielectric 21 in the inner trenches 11 and so may be formed in the sameprocessing steps. FIG. 2 illustrates a difference in the dielectrics 21and 28. The dielectric 28 in the perimeter trench 28 of FIG. 2 isthicker than the dielectric 21 in the inner trenches 11. Thus, thedielectrics 28 and 21 may be separately optimised in composition andthickness for their separate field-effect actions at the perimeter ofthe device and within the rectifier array. Each dielectric layer 21 or28 may be of substantially uniform thickness, as illustrated in FIGS. 1and 2. However, the thickness of the dielectric 21 and/or 28 may varywith depth so as to tailor the field effect action with depth.

[0031]FIGS. 3 and 4 illustrate rectifiers in which the dielectricmaterial 21 that lines the inner trenches 11 is of increased thicknessin the substrate-adjacent portion of the drift region as compared withits thickness in the surface-adjacent portion. Thus, the dielectricportion 21 x lining the upper portion of the trench 11 is thinner thanthe dielectric portion 21 y lining the lower portion. Such a variationin dielectric thickness can be particularly beneficial when the driftregion 4 has distinct surface-adjacent and substrate-adjacent portions 4x and 4 y, respectively, with distinctly different doping concentrationsN− and N. It can also be of benefit when the trenches 11 reach to thehighly-doped substrate 60. Such situations are illustrated in FIGS. 3and 4.

[0032] In the rectifiers 1 c and 1 d of FIGS. 3 and 4, thesurface-adjacent portion 4 x has a lower doping concentration N− thanthe doping concentration N of the substrate-adjacent portion 4 y. Thesurface-adjacent portion 4 x may have a low uniform doping concentrationN− of, for example, 10¹⁵ or 10¹⁶ cm⁻³. The substrate-adjacent portion 4y may also have a uniform doping concentration N, for example, of 10¹⁷cm⁻³. However, the substrate-adjacent portion 4 y may have a gradeddoping concentration N that increases with distance to the substrate.Thus, the doping concentration N of the drift region portion 4 y mayincrease from, for example, 1×10¹⁶ cm⁻³ adjacent to the portion 4 x to,for example, 3×10¹⁷ cm⁻³ adjacent to the interface with the substrate60. Similarly, the substrate-adjacent portion 4 y of the drift region 4of the devices of FIGS. 1 and 2 may have a graded or increased dopingconcentration N.

[0033] So as to reduce the capacitive coupling at the bottom of thetrenches, the dielectric material 21 y and 28 adjacent to the increaseddoping concentration (N of portion 4 y and/or n+ of substrate 60) ispreferably made thicker than the dielectric layer 21 x adjacent to thelower doping concentration (N− of portion 4 x). Such a situation isillustrated in FIGS. 3 and 4. The dielectric material 28 that lines atleast the lower part of the perimeter trench 18 is of the samecomposition and thickness as the composition and increased thickness ofthe dielectric material 21 y of the inner trenches 11. In the FIG. 3device, the dielectric 28 is of the same thickness in both thesurface-adjacent portion 4 x and the substrate-adjacent portion 4 y ofthe drift region. In the FIG. 4 device, the dielectric 28 y is thickerin the substrate-adjacent portion 4 y than the portion 28 x in thesurface-adjacent portion 4 x.

[0034] In the devices of FIG. 1 to 3, the perimeter field plate 38 is anextension of the main electrode 33. The thick dielectric layer 28 linesthe perimeter trench 18 throughout its depth. FIG. 4 illustrates adifferent situation in which the inner trenches 11 may run into thedeeper and wider perimeter trench 18. The rectifier of FIG. 4 has itsperimeter field plate 38 formed by an extension of the innertrench-electrode 31. Thus, the thinner and thicker dielectric portions28 x and 28 y in the perimeter trench 18 of FIG. 4 may be formed in thesame process steps (with the same composition and thickness) as thethinner and thicker dielectric portions 21 x and 21 y in the innertrenches 11.

[0035] The increased doping and dielectric thickness in the field-reliefstructures of FIGS. 3 and 4 can be useful for fabricating Schottkyrectifiers 1 c and 1 d having a low leakage current, a relatively highbreakdown voltage and a low on-resistance.

[0036] By way of example, FIGS. 1 and 3 illustrate rectifiers 1 a and 1c in which the drift region 4 extends to the surface 10 a between theoutermost inner trench 11 and the perimeter trench 18. An activerectifier area 43 a is formed with the Schottky electrode 3 in this areabetween the outermost inner trench 11 and the perimeter trench 18.Instead of an active rectifier area 43 a, it is also possible to provideother features in this area adjacent to the perimeter trench 18. Thus,for example, a breakdown shielding region 25 such as described in theU.S. application Ser. No. 09/167,298 and column 11 of U.S. Pat. No.5,998,833 may be provided in at least a part of this perimeter area.FIGS. 2 and 4 illustrate the inclusion of such a region 25, which is ofopposite conductivity type (p-type) to that of the rectifier driftregion (n-type) which may be much more highly doped (p+). The region 25is contacted by the electrode 3 which forms an ohmic contact therewith.The region 25 forms a p-n junction 45 with the area 4 b of the driftregion 4. The p-n junction 45 can function as an avalanche diode thatturns on at the breakdown voltage. Both FIG. 2 and FIG. 4 illustratethis p-n junction 45 as terminating in the thick dielectric layer 28 or28 y of the perimeter trench 18.

[0037] The rectifiers of FIGS. 1 to 4 can be manufactured using knowntechnologies. The device structures of FIGS. 1, 3 and 4 can bemanufactured without requiring additional masking and processing stepsto fabricate the perimeter field-plate structure 38,28,18.

[0038] Thus, FIG. 5 illustrates a manufacturing stage, in which theinner trenches 11 and a wider, deeper perimeter trench 18 are etchedinto the semiconductor body 10 using the same process steps and viarespective windows 58 and 51 in a masking pattern 50 on the surface 10 aof the body 10. The windows 51 for the inner trenches 11 are so narrowas to restrict the etch rate for these trenches 11 as compared with awider window 58 for the perimeter trench 18. Thus, this process exploitsto its advantage the well-known phenomenon of a so-called “loadingeffect”, in which the etch rate is dependent on the amount of etchablesurface exposed to the etchant.

[0039] In the devices of FIGS. 2 to 4, the perimeter trench 18 extendsto the perimeter 15 of the body, and so the individual device bodiesmanufactured side-by-side in the wafer share a common double-widthtrench 18,18′ around their individual perimeters. In this case, theseparate bodies 10 are formed at a final stage in manufacture, bydividing the wafer along scribe-lanes 55 along the field-plate structurein the bottom of the common double-width trench 18,18′. Correspondingparts of the neighbouring device body in FIG. 5 are given the samereference signs as those of the body of FIGS. 1 to 4, but followed by anapostrophe.

[0040] Vertical discrete devices 1 a, 1 b, 1 c and 1 d have beendescribed with reference to FIGS. 1 to 4, having their second mainelectrode 34 contacting a substrate 60 at the back surface 10 b of thebody 10. However, an integrated device is also possible in accordancewith the invention. In this case, the highly conductive region 60 may bea doped buried layer between a device substrate and an epitaxial region4 and may be contacted by electrode 34 at the front major surface 10 avia a doped peripheral contact region which extends from the surface 10a to the depth of the buried layer.

[0041] From reading the present disclosure, other variations andmodifications will be apparent to persons skilled in the art. Suchvariations and modifications may involve equivalent and other featureswhich are already known in the design, manufacture and use ofsemiconductor devices, and which may be used instead of or in additionto features already described herein.

[0042] Although claims have been formulated in this Application toparticular combinations of features, it should be understood that thescope of the disclosure of the present invention also includes any novelfeature or any novel combination of features disclosed herein eitherexplicitly or implicitly or any generalisation thereof, whether or notit relates to the same invention as presently claimed in any claim andwhether or not it mitigates any or all of the same technical problems asdoes the present invention.

[0043] The Applicants hereby give notice that new claims may beformulated to any such features and/or combinations of such featuresduring the prosecution of the present Application or of any furtherApplication derived therefrom.

1. A Schottky rectifier comprising a semiconductor body having a bodyportion of one conductivity type between first and second mainelectrodes, of which the first main electrode forms a Schottky barrierwith the body portion at a plurality of rectifier areas of a firstsurface of the body portion, and a pattern of trenches extending intothe body portion from the first surface, the pattern comprising innertrenches that bound each rectifier area and a perimeter trench that hasan inside wall extending around the outer perimeter of the plurality ofrectifier areas, the trenches accommodating a field-electrode that isconnected to the first main electrode, the field-electrode beingcapacitively coupled to the body portion via dielectric material thatlines the trenches so as to provide field-relief regions in the bodyportion, a depletion layer being formed in the body region from theSchottky barrier and from the field-relief regions in a blocking stateof the rectifier, characterised in that the field-electrode in theperimeter trench is present on dielectric material on said inside wallof the perimeter trench and is capacitively coupled across said insidewall without acting on any outside wall, and in that the inner andperimeter trenches are sufficiently closely spaced and the intermediateareas of the body portion are sufficiently lowly doped that thedepletion layer formed in the body portion in the blocking state of therectifier depletes the whole of the intermediate areas of the bodyportion between the trenches at a voltage less than the breakdownvoltage.
 2. A rectifier as claimed in claim 1 , further characterised inthat the perimeter trench extends deeper in the body than the innertrenches.
 3. A rectifier as claimed in claim 2 , further characterisedin that the body portion comprises a drift region present on a higherconductivity substrate of the same one conductivity type, and theperimeter trench extends through the thickness of the drift region tothe substrate.
 4. A rectifier as claimed in claim 1 or claim 2 or claim3 , further characterised in that the perimeter trench is wider than theinner trenches.
 5. A rectifier as claimed in claim 4 , furthercharacterised in that the perimeter trench extends to the perimeter ofthe semiconductor body and so provides no outside wall.
 6. A rectifieras claimed in any one of claims 1 to 5 , further characterised in thatthe dielectric material that lines at least a lower part of theperimeter trench is thicker than the dielectric material that lines atleast an upper part of the inner trenches.
 7. A rectifier as claimed inany one of claims 1 to 6 , further characterised in that the bodyportion comprises a drift region present on a higher conductivitysubstrate of the same one conductivity type, and the drift region hasdistinct surface-adjacent and substrate-adjacent portions which are ofdifferent doping concentrations, the surface-adjacent portion having alower doping concentration than the substrate-adjacent portion.
 8. Arectifier as claimed in any one of claims 1 to 7 , further characterisedin that the body portion comprises a drift region present on a higherconductivity substrate of the same one conductivity type, and thesubstrate-adjacent portion of the drift region has a graded dopingconcentration that increases with distance to the substrate.
 9. Arectifier as claimed in claim 7 or 8 , further characterised in that thedielectric material that lines the inner trenches is of increasedthickness in the substrate-adjacent portion of the drift region ascompared with its thickness in the surface-adjacent portion.
 10. Arectifier as claimed in claim 9 , further characterised in that thedielectric material that lines at least a lower part of the perimetertrench is of the same composition and thickness as the composition andincreased thickness of the dielectric material of the inner trenches inthe substrate-adjacent portion of the drift region.
 11. A rectifier asclaimed in any one of claims 1 to 10 , further characterised in that thebody portion of the one conductivity type extends to the surface betweenthe outermost inner trench and the perimeter trench.
 12. A rectifier asclaimed in any one of claims 1 to 11 , further characterised in that abreakdown shielding region of the opposite conductivity type is presentbetween the outermost inner trench and the perimeter trench and forms ap-n junction with the body portion of the one conductivity type.
 13. Amethod of manufacturing a rectifier as claimed in claim 4 , wherein thewider, deeper perimeter trench and the inner trenches are etched intothe semiconductor body using the same process steps and via respectivewindows in a masking pattern on the surface of the body, and wherein thewindows for the inner trenches are so narrow as to restrict the etchrate for the array trenches as compared with a wider window for theperimeter trench.